A Pomeranchuk cell which is precooled and pressurized by a He4-cycling He3 - He4 dilution refrigerator

This paper deals with a special Pomeranchuk cooling device which is used in combination with a He⁴-cycling He³-He⁴ dilution refrigerator at high pressures. The lowest temperature obtained was 1.1 mK.     

Achieving ultralow temperatures by classical He3-He4 dilution refrigeration

It is now possible to achieve temperatures at least as low as 2.0 mK in a continuous way by dilution of He³ in He⁴ with the use of continuous Ag powder heat exchangers of very large area. The equation of a continuous heat exchanger is rewritten in a practical form. This equation allows the calculation of the area needed to achieve a given minimum temperature as a function of flow rate, heat leaks and Kapitza resistance.Heat leaks, and on the calculation of the heat exchanger channels sizes are discussed.     

Determination of thicknesses and impurities of targets by measuring backscattered particles from 2 MeV 4He+-bombardment

The measurement of target thicknesses and impurity contents using backscattering, as well as the principle of this technique are briefly described. The targets are irradiated in a beam of 2 MeV ⁴He⁺ ions. The backscattered ions are detected using a Si(Au) solid-state detector. Gold and calcium fluoride on thin carbon backing and selfsupporting silicon foils were determined from peak width measurements. Oxygen and tungsten impurities in a silicon target being prepared by vacuum evaporation from a tungsten boat were determined using the same method. The thickness of a thin aluminium deposit on thick tantalum backing was obtained from the energy shift of the tantalum peak between two spectra recorded with respectively the aluminium and the tantalum facing the ion beam. Copper contamination on the surface on an iron layer electroplated on copper foil was determined by comparison of the copper peak area with the iron peak height in the backscattering spectrum.     

A cryostat for investigating inelastic scattering of neutrons on liquid He4 between 4.2 and 0.5 K

A cryostat is described which is designed for the research of inelastic scattering of neutrons on a large volume (3.5 l) of He⁴ between 4.2 and 0.5 K. Temperatures lower than 1.3 K are produced by means of pumping out vapour above liquid He³ (～ 330 cm³) with an adsorption pump (1 kg of coal CKT-2) located in the device itself. A minimum temperature is held during 140 h. The essential time for the cryostat start is about 10 h. The cryostat was used for the measurement of temperature dependence of the Bose-condensate density in He⁴.     

Surface planarization and masked ion-beam structuring of YBa2Cu3O7 thin films

Surface planarization and masked ion-beam structuring (MIBS) of high-T_c superconducting (HTS) YBa₂Cu₃O_7-δ (YBCO) thin films grown by pulsed-laser deposition (PLD) method is reported. Chemical-mechanical polishing, plasma etching, and oxygen annealing of YBCO films strongly reduce the particulate density (~ 10^-2 ×) and surface roughness (~ 10^-1 ×) of as-grown PLD layers. The resistivity, critical temperature T_c ≈ 90 K and critical current density J_c (77 K) > 1 MA/cm² of films are not deteriorated by the planarization procedure. The YBCO films are modified and patterned by irradiation with He⁺ ions of 75 keV energy. Superconducting tracks patterned by MIBS without removal of HTS material and, for comparison, by wet-chemical etching show same T_c and J_c(T) values. Different micro- and nano-patterns are produced in parallel on planarized films. The size of irradiated pattern depends on the mask employed for beam shaping and features smaller than 70 nm are achieved.     

In Situ Mitigation of Subsurface and Peripheral Focused Ion Beam Damage via Simultaneous Pulsed Laser Heating

Focused helium and neon ion (He⁺/Ne⁺) beam processing has recently been used to push resolution limits of direct‐write nanoscale synthesis. The ubiquitous insertion of focused He⁺/Ne⁺ beams as the next‐generation nanofabrication tool‐of‐choice is currently limited by deleterious subsurface and peripheral damage induced by the energetic ions in the underlying substrate. The in situ mitigation of subsurface damage induced by He⁺/Ne⁺ ion exposures in silicon via a synchronized infrared pulsed laser‐assisted process is demonstrated. The pulsed laser assist provides highly localized in situ photothermal energy which reduces the implantation and defect concentration by greater than 90%. The laser‐assisted exposure process is also shown to reduce peripheral defects in He⁺ patterned graphene, which makes this process an attractive candidate for direct‐write patterning of 2D materials. These results offer a necessary solution for the applicability of high‐resolution direct‐write nanoscale material processing via focused ion beams.     

Copper diffusion in thin In2S3 layers investigated by Rutherford Backscattering Spectroscopy

Copper diffusion in thin In₂S₃ layers was investigated by measuring copper concentration profiles with Rutherford Backscattering Spectroscopy (RBS). For this purpose In₂S₃ layers (thickness 100 nm) were evaporated on crystalline silicon substrates and Cu was diffused from a CuSCN surface layer at temperatures between 150 and 250 °C. The CuSCN source layer was removed in pyridine solution before RBS measurements were performed. Diffusion coefficients of Cu in In₂S₃ were obtained by simulating the diffusion process in a one-dimensional model and comparing measured and simulated Cu concentration profiles. The values of the exponential prefactor of the diffusion coefficient and of the activation energy were 9 × 10^− 11 cm²/s and 0.3 eV, respectively.     

He+ backscattering and Auger electron spectroscopy study of the ion-induced modification of copper films deposited on single-crystal silicon

The sputtering yields of copper due to argon ion milling were measured by mega-electronvolt He⁺ Rutherford backscattering and Auger electron spectroscopy for thin copper films deposited on 〈111〉-oriented silicon substrates. We found that recrystallization occurs in the copper films during the argon ion bombardment. The copper profile broadens at the Si-Cu interface and the spread increases with the beam energy during ion milling.     

The properties of ion-implanted LiNbO3 waveguides measured by the RBS and ion beam etching stripping methods

Z-cut LiNbO₃ crystals were implanted at room temperature with 3.0 MeV O ions with a fluence of 6 × 10¹⁴ ions/cm². The effective refractive index of the guided mode was obtained by the prism coupling technique combined with mechanical stripping (ion beam etching) at room temperature. Surface layers, with thicknesses of 300, 630, 880, 1100, and 1500 nm, were removed. The channel spectra of LiNbO₃ waveguides with different etching depths were measured by the RBS technique. The damage profile of the LiNbO₃ waveguide was studied by dechannelling analysis with a multiple scattering function based on the channel spectrum. The refractive index profile was calculated based on the lattice damage ratio. The effective refractive index of the guided mode simulated by BPM showed good agreement with the experiment value.     

Diffusion of Pb in (100) Si under electron beam annealing following dual ion implantations of Pb/Ne, Pb/O and Pb/N

(100) Si was dual-implanted with the ions Pb⁺/²²Ne⁺ (7 and 30 keV), Pb⁺/¹⁶O⁺ (7 and 26 keV) and Pb⁺/¹⁴N⁺ (7 and 24 keV) to peak concentrations of typically 10 at.%. The implanted samples were then electron beam annealed at 900 °C for 30 s with a temperature gradient of 5 °C s⁻¹ under high vacuum conditions. Channelled RBS measurements performed with 1.5 MeV ⁴He⁺ ions showed that annealing of the Pb/Ne and Pb/O samples resulted in an almost complete recrystallisation of the amorphous layers caused by the ion implantations and a total loss of the implanted Pb. For the Pb/Ne samples the Ne diffused out to leave a rough surface sprinkled with deep craters; for the Pb/O samples some SiO₂ formed below the surface. In contrast, for the Pb/N samples most of the amorphous layer survives annealing and almost all the Pb is retained. A striking feature is that annealing causes the Pb to diffuse away from the surface to be trapped in a deep diffusion sink provided by the implanted N. XRD analyses exhibited Pb (111) and Pb (220) reflections suggesting that Pb nanoclusters have grown in the understoichiometric silicon nitride layer. These structures offer an interesting opportunity for controlled carbon nanotube (CNT) growth on silicon nitride.     

Diffusion Monte Carlo Study of the Small Mixed 3He-4He Clusters

The ground state properties of helium mixed clusters ³He_2,3,4 ⁴He₂, ³He_2,3,4 ⁴He₃, ³He_2,3,4 ⁴He₄, ³He_2,3 ⁴He₅, consisting of up to eight helium atoms are studied using variational and diffusion Monte Carlo calculations. For clusters with three and four ³He atoms released-node diffusion Monte Carlo method is used. Our calculations show that, within errorbars, clusters ³He_3,4 ⁴He₂ have the same binding energy as the cluster ³He₂ ⁴He₂, and that ³He₃ ⁴He₃ has the same binding energy as ³He₂ ⁴He₃. The clusters ³He_3,4 ⁴He₂ and ³He₃ ⁴He₃ are in states in which one or two ³He atoms are far away from the rest of the system. Other considered clusters are bound. In particular, we have shown the stability of the cluster ³He₄ ⁴He₃, which was previously considered unstable. The calculations are performed using several different interatomic potentials and the conclusions concerning stability are insensitive to the particular form of the interaction potential. We compare our results with the recent experiment and other theoretical calculations.     

16O contamination in 4He analysis beams

The ⁴He⁺ beam from a Van de Graaff accelerator is often accompanied by an ¹⁶O⁺ beam of the same energy. If, after acceleration and before magnetic analysis, one electron is stripped from the oxygen ion to form ¹⁶O²⁺, these ions will not be separated from ⁴He⁺ during magnetic analysis. The ¹⁶O²⁺ fraction in the ⁴He⁺ beam was measured by analyzing back-scattering spectra for a thin Au film and the O to He ion charge ratio of 2:1 was confirmed by electrostatic deflection. Ion source conditions and the pressure in the beam lines strongly affect the ¹⁶O²⁺ fraction and the dependence of the O²⁺/O⁺ ratio on pressure is found to be in approximate agreement with predicted values. A formula for estimating the ¹⁶O²⁺ beam intensity, based on charge exchange data and measurement of the primary ¹⁶O⁺ beam intensity, is N[O²⁺] = 13 P(torr) l(cm) N[O⁺], where P is the total gas pressure, l the length of the beam line between the accelerator and switching magnet and N[O²⁺] and N[O⁺] are the beam intensities of the respective species. The ¹⁶O²⁺ contamination of ⁴He⁺ beams is easily removed by electrostatic separation.     

Solid-state field-assisted silver diffusion in (TeO2)0.6(WO3)0.25(La2O3)0.05(Na2O)0.1 glass

Silver-doped layers have been produced in (TeO₂)_0.6(WO₃)_0.25(La₂O₃)_0.05(Na₂O)_0.1 (TWLN) glass by solid-state field-assisted diffusion. The silver concentration profile in the glass has been determined by secondary ion mass spectrometry (SIMS) and Rutherford backscattering spectrometry (RBS). The Matano-Boltzmann method applied to thermally activated diffusion indicates that the silver diffusion coefficient in the glass is a weak function of silver concentration. We carried out modeling of silver concentration profiles in the doped layer of the TWLN glass after solid-state field-assisted diffusion. Good agreement of the theoretical fit and experimental data suggests that the model chosen can be used to describe solid-state field-assisted diffusion of silver ions in TWLN glasses. Using RBS data, we have quantitatively estimated the surface density of silver atoms after field-assisted diffusion. Combining SIMS and RBS, we were able to assess the absolute silver concentration depth profile. Our results demonstrate that field-assisted diffusion, a simple and effective procedure for doping optical glasses, can be used to produce planar waveguide structures based on the tellurite glass studied here.     

Low-temperature heat capacity of helium films adsorbed on copper

Heat capacity measurements are described for films of pure⁴ He and a 12%He ³ mixture ofHe ³ andHe ⁴He adsorbed on copper, for 0.1<T<1.2 K.He ⁴ heat capacity isotherms show a step-type behavior as a function of coverage for the first two layers, while the mixture isotherms only show the first step. A comparison is made with recent multilayer data for helium films on Vycor published by other authors.Work supported by the National Science Foundation.     

Ion bombardment of polyethylene—influence of polymer structure

Polyethylenes of various macromolecular and supermolecular structures were studied from the point of view of their susceptibility to an ion beam treatment. An influence of molecular weight (M_w), molecular weight distribution (M_w/M_n) and the degree of branching were compared within the set of low-density polyethylenes (LDPE) studied. An influence of the length of branches was compared between LDPE, linear low-density (LLDPE) and high-density (HDPE) polyethylenes. An influence of the degree of crystallinity and the morphology of a crystalline phase were compared for HDPE samples solidified under various thermal conditions and ultra-high molecular weight polyethylene (UHMWPE). Plate polymer targets ∼2 mm were bombarded with 100 keV He⁺ or 130 keV Ar⁺ ions (dose of 10¹⁴-10¹⁶ ions/cm²; ion energy stream density <0.1 μA/cm²), micromechanical properties of their surface layer (hardness, mechanical modulus and elastic recovery) determined and compared to the virgin materials.Ar⁺ ion beam bombardment generally lowers micromechanical properties of the polyethylenes, whereas He⁺ ion beam treatment makes them higher. The effect is the stronger the higher the molecular weight of polyethylene. However, a long chain branching adversely affects the modification. The degree of crystallinity facilitates an ion beam bombardment from the point of view of micromechanical properties of the materials, however, also the morphology of a crystalline phase was found to play a role.     

Diffusion process applied in fabrication of ion-exchanged optical waveguides in novel Er3+ and Er3+/Yb3+-doped silicate glasses

Planar optical waveguides fabricated in erbium doped glasses are promising photonics components operating at 1.5 μm, above all as optical amplifiers or waveguide lasers. In this study we focused on the influence of chemical compositions of Er³⁺ and Er³⁺/Yb³⁺-doped silicate glasses on the properties important for the sought applications, i.e., waveguiding and spectroscopic properties of the fabricated waveguides. Optical waveguides were fabricated by routine K⁺ ↔ Na⁺ ion exchange in a set of the glasses that had various contents of the RE ions. Waveguiding properties were studied by means of Dark Mode Spectroscopy at 671 nm while the chemical composition of the waveguides was determined using Scanning Electron Microscopy (SEM-EDAX). Surface concentrations of the lasing RE ions were determined by Rutherford Backscattering Spectroscopy (RBS) and related to the resultant photoluminescence properties. On the bases of the obtained experimental results the relations between composition of the substrates, especially content of the active ions, and experimental procedures will be presented with regards to real fabrication processes. Diffusion processes will be characterized by diffusion coefficients.     

Electron-beam-induced modification of stoichiometry and acceleration of titanium diffusion in Al2O3/Al/C structures

The electron-beam-induced modification of stoichiometry and acceleration of titanium diffusion in Al₂O₃/Al/C structures have been studied for the first time using He⁴⁺ ion backscattering, scanning electron microscopy, and electron probe microanalysis techniques. It is established that the electron beam treatment leads to an increase in the effective diffusion coefficient of ion-implanted titanium, changes the ratio of aluminum and oxygen concentrations in the α-Al₂O₃ layer, and induces the substitution of carbon for oxygen in this layer.     

He4 film flow suppressor for a dilution still and He3 purification

The design and construction of a still for a dilution refrigerator is described. This device yielded an He³He⁴ ratio of almost 300. In addition to its use in a dilution refrigerator the still has been used for He³ purification, where it proved capable of extracting a high proportion of He³ from gas with a large He⁴ contamination.     

Dynamic Surface Properties of Liquid 3He Probed by Surface State Electrons

Two-dimensional (2D) electrons floating on the liquid He surface is a powerful tool to study the dynamic surface properties of both normal and superfluid ³ He. From the mobility measurement an evidence is found for the existence of ripplons on liquid ³ He at least above 140 mK. When the 2D electrons form a Wigner solid (WS) at low temperatures, they are accompanied with surface depression under each lattice site. Low frequency mobility of the WS probes the scattering of quasiparticles from the surface depression. The damping of high frequency magnetophonon resonances of the WS is dominated by the scattering of electrons from surface excitations. A comparative measurement on both ³ He and ⁴ He shows a prominent difference in the surface properties of ³ He from ⁴ He below 70 mK.     

Mixing in Au/Ge system induced by Ar<Superscript>+</Superscript> ion irradiation

Rutherford Backscattering Spectrometry (RBS) and Electrical Resistivity Measurements (ERM) were used to investigate the mixing of Au/Ge bilayer deposited onto glass substrate induced by Ar ions. Mixing was initiated by bombarding the sample with 400 keV ⁴⁰Ar⁺ beam with a fluence up to 1.2 × 10¹⁷ ions/cm² at a constant flux of 0.25 μA/cm². To assist the evaluation of the experimental results, all spectra were simulated using “RUMP” computer code. RBS results indicated that ion beam mixing led to a formation of AuGe₂ compound. The mixed region was noticed to increase with the gradual increase of Ar⁺ fluence. Results were also compared with current theoretical models used to describe the mixing process. The Bφrgesen thermal spike model was found to accurately predict the diffusion in Au/Ge interface. An increase in the electrical resistivity of the film was detected during Ar⁺ irradiation.